Substituent effects 1,5-sigmatropic shift

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

The kinetics of [1,5] hydrogen shifts in some alkyl-substituted (Z)-l,3-pentadienes have been determined23 50-52. From kinetic analyses53,54 of substituent effects on the reaction, several trends are evident. In the temperature range studied (130-250 l C), the [1,5] sigmatropic hydrogen shift is usually reversible. Only if there is an appreciable thermodynamic stabilization of the product is the reaction found to be unidirectional (Table 2, entries 7, 13, 16). 

Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,-diphenyl-l,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene. Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl. These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5-hexadiene. 

The substituent effects on [ 1,5]sigmatropic ester shift (68) have been scrutinized. The increases with the hardness of the substituent R geminal to the... 

The somewhat low reactivity of ( )-phenylsulfonyl-2-trimethylsilylethylene in Diels-Alder reactions is probably due to steric hindrance exerted by both substituents and by the poor activation imparted by the silyl group. This drawback is partially offset by the effective elimination to the alkene performed under very irdld conditions with fluoride ion. The low dienophilic reactivity of the title reagent is evident in the reaction with isodicy-clopentadiene, for which it was demonstrated that only the isomer arising from the [l,5]-hydrogen sigmatropic shift was captured by dienophiles of low reactivity. Flighly reactive dienophiles react... 

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